This invention relates generally to electronic circuits and more specifically, to a frequency hopping oscillator circuit that may be utilized in, for example, a phase locked loop (PLL).
It is common to utilize a PLL in conjunction with a voltage controlled oscillator to coherently provide a local oscillator signal to a receiver and transmitter circuit within a transceiver. A PLL typically utilizes a reference oscillator which is used as a reference frequency to “lock” the output local oscillator (LO) at a specific frequency. The reference oscillator is typically of a high quality crystal type which provides a low phase noise reference to the PLL. Various types of crystal oscillators exist, including oven-controlled crystal oscillators (OCXO), temperature-controlled crystal oscillators (TCXO), and voltage controlled crystal oscillators (VCXO). The VCXO reference frequency can be fine-tuned by adjusting a DC control voltage. Using any of the above described reference oscillators is common practice in PLL circuits.
A problem arises when two or more narrow-band coherent transceivers are in close proximity. If the transmit signals from those transceivers are at the same frequency, they may adversely interfere with one another. One current method to reduce the interference is for each transceiver to randomly change frequency within a bandwidth. Each of these frequency changes is commonly referred to as a frequency hop, and multiple frequency changes are referred to as frequency hopping. A digital circuit provides digital data to the PLL in order to initiate such a frequency hop. Increasing the number of frequency hops within a fixed bandwidth improves the interference rejection, however, when using currently available phase locked loop oscillator circuitry for the local oscillator, increasing the number of frequency hops degrades the phase noise of the oscillator such that an upper limit of discrete frequencies within the bandwidth cannot be exceeded, therefore limiting the amount of interference that can be rejected.
With known frequency hopping oscillator circuits, there are limitations on the number of discrete frequency hops that can be created within a fixed bandwidth without degrading the phase noise.